1. Because you actually need protein to gain muscle mass, so there goes your "mostly plants" without a caveat about "vegetarian protein", which has its own set of problems.

2. Exercise regularly - what is regularly? What kind of exercise? You never had over-work injuries? Injuries from muscle / postural imbalances? Etc.

I concede that most of this advice is available for free, but unless you have a GOOD coach, you will get injured before you get results. Then there is plain doing too much.

Again, you can come back with "Well, I am just talking about not being obese" - in that case, yea, it's much simpler since you don't have to worry about getting enough protein and balancing that against becoming over-weight.